Changes from HCP

Hi There,

The Human Connectome Project has been using a modified version of gradunwarp for some time now.

I've been using some of their code, and the multiple versions of gradunwarp have given rise to some confusion and installation difficulties. So I asked them if they'd be alright with me submitting a pull request including a backwards compatible version of the changes, and got the go ahead. So here's that.

Please don't hesitate to get in touch about the changes. I did not personally write non_linear_unwarp_siemens, so don't have much to say about the decisions it makes, but can find the person who did if you have questions for them. This method only gets called if you provide the --hcp flag to the command line, so none of the current users should notice a change.

Ben